1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus which provides power of an electric motor to the steering system of a vehicle to thereby reduce the manual steering effort or force needed to turn the steering wheel.
2. Description of the Related Art
Electric power steering apparatuses are known in which a steering torque produced in conjunction with a manual steering operation taken by the driver is detected by a steering torque sensor, a steering torque signal supplied from the steering torque sensor is converted into a target current signal, an offset or deviation between the target current signal and a motor current signal corresponding to a current flowing actually in the electric motor is corrected by PI (proportional plus integral) or PID (proportional plus integral plus derivative) control action to generate a corrected signal, and a motor control signal (such as a pulse-width modulation signal) generated on the basis of the corrected signal is used to drive the electric motor to apply a steering assist force or torque to the steering system of a vehicle.
In the known electric power steering apparatus, because the PI or PID control action is carried out while the motor current signal is fed back to the target current signal to rapidly reduce the offset between these two signals to zero (negative feedback), the motor current becomes equal in quantity to the target current to thereby enable the electric motor to provide a steering assist torque corresponding to the steering torque signal.
However, when the steering wheel is quickly turned in either direction until it reaches a rack end position (steering lock position), rotation of the electric motor is suddenly stopped with the result that a counterelectromotive force (counter emf.) of the electric motor disappears suddenly and a current corresponding to the disappeared counter emf. is added to the motor current, tending to cause overshoot of the motor current.
An excessive current (overcurrent) produced due to the motor current overshoot may exceed a maximum rating current of semiconductor devices used in a motor drive unit as switching elements for the PWM (pulse-width modulation) driving of the electric motor and thereby damages the semiconductor devices.